328 AXEL A. OLSSON 



Shell elongated trigonal, the umbones median, moderately convex, thin. 

 Posterior-dorsal area widely flattened but with a small mid-rib, the outer 

 keel generally high and winged. Valves closed, no posterior gap. 



Length 78.7 mm., height 56 mm., semidiameter 16 mm. a right valve, 

 Bucaro, Panama. 



M. alata has generally been considered a West Indian or Caribbean 

 species strictly, but it is now known to be fairly common at several places 

 along the Pacific Coast. A typical specimen was figured by Reeve from 

 Santa Elena, Ecuador; Morch in 1861, proposed the name "subalata" from 

 a single right valve found at Realejo, near Corinto, Nicaragua, but the 

 characters selected by March as distinctive are of doubtful significance. 

 As a fossil, M. alata has been recorded from the Miocene of the Dominican 

 Republic and Costa Rica. 



Range — Nicaragua to Ecuador. Panama: Burica Peninsula; Guanico; 

 Bucaro. Ecuador: Sau; Santa Elena. 



Mactrellona cUsea (Dall) Plate 56, figure 1 



Mactrella clisea Dall, 1915, Nautilus, vol. 29, No. 6, pp. 62, 63 "no mention of locality." 

 — Dall, 1916, Proc. U.S. Nat. Museum, vol. 52, p. 415 type stated as from 

 Manzanilio, Mexico. 



Mactra {Mactrella) clisea (Dall), Olsson, 1935, Nautilus, vol. 48, No. 3, p. 105.— Max- 

 well Smith, 1944, Panamic Marine Shells, p. 66, fig. 860B. 



Mactrella {Mactrella) clisea Dall, Hertlein and Strong, 1950, Zoologica, vol. 35, pt. 

 4, p. 233. 



Mactrellona {Mactrellona) clisea (Dall), Hertlein and Strong, 1955, Bull. Amer. Mus. 

 Nat. Hist, vol. 107, art. 2, pp. 204, 205. 



Shell medium to large, thin, arcuate, hatchet-shaped, the zone of 

 greatest inflation extending from the beak to the posterior-middle side 

 of the ventral margin which is widely bulged as a result. The anterior side 

 is somewhat produced, narrowed and depressed. The posterior-dorsal area 

 is wide, flattened and carries a small mid-rib; externally, it is bordered by 

 a raised, frilled or winged keel. There is a small gap near the forward end 

 of the posterior area. Surface roughened by crowded growth lines and 

 covered by a thin, yellowish periostracum. 



The largest specimen measures: length 85 mm., height 70 mm., semi- 

 diameter 20 mm. a right valve. 



Distinguished from M. alata by its higher, arcuate form, and the 

 presence of a small, posterior gap. 



Range — Gulf of California to Ecuador. Panama: Bucaro; Guanico; 

 Burica. Ecuador: Sua; Galeras; Canaoa; Manglaralto; Santa Elena. 



Mactrellona exoleta (Gray) Plate 58, figures 3-3b 



Mactra exoleta Gray, 1837, Mag. Nat. Hist., vol. 1, p. 372 no locality cited. — Hanley, 

 1843, Cat. Recent Bivalve Shells, p. 33; 1856, p. 340, pi. 11, fig. 51.— Reeve, 

 1854, Conch. Icon., vol. 8, Mactra, pi. 4, fig. 16. — Grant and Gale, 1931, Mem. 

 San Diego Soc. Nat. Hist., vol. 1, p. 402, pi. 22, figs. 10a, 10b. 



Mactrella {Mactrella) exoleta (Gray), Hertlein and Strong, 1950, Zoologica, vol. 35, 

 pt. 4, p. 234. 



Shell of medium or large size, thin, with large, full umbones and evenly 

 inflated valves. The posterior-dorsal area is wide, flattened or gently 

 vaulted, externally limited by an angled line or low ridge which is not 

 winged or frilled. The texture of the shell is thin and fragile, marked ex- 



